Thyroid hormone appear to act by binding to receptor proteins present in chromatin. The major focus of the proposed studies is to purify these receptors after solubilizing them from chromatin. Most of the effort will be devoted to affinity chromatography, since studies to date suggest that this methodology will be successful. We also plan to use other conventional approaches and to explore the use of other derivatives of thyroid to enhance the purification. Our studies to date suggest that the binding properties of the biologically active form of the receptor are generated by protein-protein interaction. Thus, with the use of purified and partially purified receptors, we hope to examine the determinants of ligand recognition by the receptor. We also plan to examine physical properties of the receptors and their biochemical properties that may relate to the mechanism of receptor action. Receptors bound by the hormone and free of hormone will also be studied for their effects on isolated nuclei (and possibly chromatin) that transcribe a thyroid hormone responsive gene (that for growth hormone). It is hoped that the work will provide one element in the thyroid hormone response that can be used ultimately with other necessary materials (e.g., pure genes etc.) in the reconstruction of a chemically defined thyroid hormone-responsive system.